I have involuntary muscle tremble

You might want to check the milestones in the wiki, but yes I'd say it's a good sign.

Sounds like you're making progress. What helped me at that early point in the process was lying very still and doing gentle PC contractions, the ones you can use to gently cut off urine flow midstream and while doing so feeling very attentively to what the anal sphincter wants to do. When you feel little wormy, fluttering contractions of the sphincter wanting to happen, try tickling your nipples and gently varying your PC contraction strength and see if you can make them intensify. Over a period of several sessions you should be able to experience involuntary contractions occurring when you do this which are mysterious, involuntary sucking in movements of the massager into the rectum against your prostate gland. It's these involuntary contractions that will over time begin to be accompanied by warm orgasmic feelings building up in your pelvis and that will eventually in sessions reach the level you feel you're actually ejaculating, though it's dry. You've just begun a journey that can take weeks to months if not in some cases longer.